The present invention relates to a thermal fixing device and an image forming device provided with the fixing device.
In an image forming device such as a laser printer, a thermal fixing device including a heat roller and a pressure roller is provided for thermally fixing a toner image onto a sheet. In the thermal fixing device, the toner image which has been transferred onto the sheet is thermally fixed to the sheet when the sheet passes through a nip between the heat roller and the pressure roller.
Generally, the heat roller installs therein a heater such as a halogen lamp, and a temperature sensor is provided for detecting a surface temperature of the heat roller. The fixing temperature can be maintained at a constant temperature by ON/OFF control to the heater based on the detection of the surface temperature by the temperature sensor.
Known as the temperature sensor are a contact type sensor such as a thermistor which is in direct contact with the heat roller and a non-contact type sensor such as an infrared radiation sensor which is out of contact from the heat roller. The non-contact type sensors have been widely used in the thermal fixing device because the heat roller can be protected against injury for a expanded duration due to out of contact from the sensor.
Conventionally, the temperature sensor has been provided in the thermal fixing device together with the heat roller and the pressure roller, because the temperature sensor must be in direct contact with the heat roller in case of the contact type sensor, or must be precisely set and oriented with respect to the heat roller in case of the non-contact type sensor. Therefore, the temperature sensor is attached to or detached from a main casing of the image forming device when the thermal fixing device is replaced by a new thermal fixing device.
In this case, the heat roller and the pressure roller must be frequently replaced by new rollers due to frictional wear as a result of pressure contact with each other, whereas the temperature sensor and particularly the non contact type sensor needs not be replaced frequently, because the latter can provide a prolonged service life because of out of contact from the heat roller. Therefore, in the conventional arrangement, frequent exchange of the thermal fixing device is not economical in terms of the sensor.